Step 1
a. Start Acronis Bootable Rescue Media Builder and make you flash drive bootable selecting all options.
b. While you have the flash drive that you just created plugged in, run Acronis and clone this drive to your external USB hard drive.
c. Since the flash drive is relatively small to the external USB hard drive, when asked you can create an additional partition under the NTFS so that it supports large 4GB+ files.
d. To boot out of this drive, don’t forget to go into the BIOS and specify that the computer should boot from this device.
e. You are done!!!!!!!!!!

I posted this because I went through hell to do this and found no solutions, so I made one up, and it worked.

Why did you want to boot to an external hard drive instead of using Acronis Startup Manager to make Acronis bootable from your local hard disk?

Personally I used this method to prepare a small ~1GB partition on my local hard disk with a bootable copy of BartPE on it (which has PE versions of Acronis, Macrium, ShadowProtect, IFW, Drive Snapshot and other programs), and bootable IFD and Drive Snapshot floppy images, and I can easily add more boot options to it.

Because of portability.
Lets say you have a netbook and a desktop and you back them up on you removable drive, upon their failure of this netbook or desktop, you can restore them from the external usb drive.
This drive will boot to Acronis, as well as contain the back up image of your computers, no additional cds' or flashdrives, or any other equipment required.